Telecommunication systems have become increasingly more complex since their inception. Modern systems are exceedingly fast and versatile, capable of carrying a multitude of different types of traffic in the same instant and over vast distances, all without human intervention. However, they necessarily imply correspondingly more complex and “intelligent” management and control systems to keep track of traffic, monitor the level of service, ensure the most efficient strategy for running the network and so on. Monitoring the network paths for errors and continuity is one specific aspect of control and management that is of particular concern.
Optical transmission is now the norm for certain parts of networks. Spans of optical fibre interconnect geographically spaced locations, often covering tens or hundreds of kilometers at a time. Recent trends in optical communication include Wavelength Division Multiplex (WDM) in which a plurality of wavelengths or wavebands are allocated to a channel, each band being capable of supporting traffic.
A typical span in an optical network is illustrated schematically in FIG. 1. An optical fibre 1 connects spaced locations A and B through one or more amplifiers, such as those indicated at 2, 3 in the drawing. These may be wholly optical or electro-optical, in which case there is a requirement for electrical-to-optical and optical-to-electrical converters in each amplifier. The transmit end A of the span typically comprises a plurality of optical input paths 4, passing through a corresponding plurality of circuits indicated generally at 5, into an optical multiplexer 6. At the receive end B an optical demultiplexer 7 splits the received optical signal into a corresponding number of receive paths 8, each with its own detector indicated generally at 9. Each input path to the multiplexer 6 and each output path from the demultiplexer 7 corresponds to one of the bands into which the signal bandwidth is divided in a WDM system.
Each link of a complex network would include one or more spans of the above general type and construction, arranged to interconnect nodes in the network, for example where the network branches. FIG. 2 illustrates schematically one such node 10 in a network at which an incoming waveband 11 from an upstream location enters from the left and exits 12 from the right on to a downstream location. It should be noted that the expressions “upstream” and “downstream” are used in the present context to illustrate a particular connection at a particular point in a network and it is not intended that the scope of the invention is to be generally restricted in any specific sense.
It is often the case that one (or more) of the wavebands making up the total signal band will be split off from the band at the node. Similarly, another band (or bands) may enter the node from another point in the network and join the signal channels directed in the downstream direction. Such an example is illustrated generally in FIG. 2. As part of the control process in an optical network, the average power in the channels is monitored at various locations throughout the network. In the present example, monitoring means could be located in the downstream channels 12 exiting the node 10.
Consider now the scenario in which an error, break or other discontinuity 13 occurs in one of the fibres constituting the upstream channels 11 entering the node. Although the channels traversing the errored or broken fibres may now not appear at the downstream node, indicating the presence of a fault, the channels inserted at 14 for onward transmission along the downstream fibre will still be present and may now display a power fluctuation or transient due to the non-ideal behaviour of the optical amplifiers along the fibre. These transients may be regarded by the communications network management system as additional, separate faults unless a method is provided to detect channel power transients and their progression between wavelength channels throughout the network. Similar transients may be caused by the deliberate introduction of channels into the network to increase capacity, and any impact on neighbouring channel powers must be detected and correlated in order that a false alarm is not raised on that neighbouring channel.
Prior art methods of monitoring channels have usually involved only average power measurement, as indicated in the preceding paragraphs. FIG. 3 is useful to illustrate one generic type of power monitoring system in which each output channel 20 from a demultiplexer 21 receiving an optical waveband 22 is connected through a respective series detector 23 in the channel. There are as many detectors as there are channels in this arrangement, commonly known as parallel scanning. This arrangement is accurate but carries a punitive cost penalty in that it requires one set of detectors or monitoring devices per channel.
In another known monitoring arrangement (not illustrated) a single scanner is arranged to be supplied with signals from each channel in turn by means of a fast serial scanning filter. However, such filters are still not fast enough to handle the high levels of traffic in such networks without causing unacceptable delays.
Monitoring the total power in all of the channels combined is fast but crude since it gives no indication as to how the individual wavelengths are affected, especially by transients.
Another technique previously employed is monitoring the eye closure of the channels. This is a measure of the amplitude response of the channel. In order to be effective, however, fast eye monitoring is necessary and it too comes with a punitive cost penalty.